• The combination of solar energy and underground hydrothermal environment supports the sustained and efficient CH4 production from CO2.
• Nanosheets of honeycomb ZnO were formed in-situ on the Co surface, resulting in a new motif (Co@ZnO catalyst) that inhibits Co deactivation through ZnO-assisted CoOx reduction.
• The stabilized Co and interaction between Co and ZnO inhibited unwanted side reaction pathways via CO production, ensuring formic acid formed as an intermediate, leading to 100% CH4 yield.
• This study presents a straightforward one-step process for both highly efficient CO2 conversion and catalyst synthesis, paving the way for solar-driven CO2 methanation.